1. Field of the Invention
This invention relates to an aluminum base alloy product. More particularly, this invention relates to an aluminum base alloy product which is weldable and further characterized by high stress-corrosion cracking resistance and by high strength making the alloy suitable for ballistics armor, including armor-piercing and fragment ballistics protection.
2. Background of the Invention
Aluminum, when used as a combination structural and armor plate material for military vehicles, must be easily weldable and have high corrosion resistance, particularly stress-corrosion cracking (SCC) resistance. In addition, of course, as armor it must possess a high strength for ballistics protection both against fragments from exploding shells and armor-piercing projectiles. While the ballistics protection is related to the thickness of the armor, weight constraints dictate that higher strength thinner armor be used in preference to thicker products made from alloys with less strength. Furthermore, the alloy must be capable of being cast or welded into various shapes and sizes, particularly when used in space frame and open frame applications.
It is known that the addition of copper and magnesium to an aluminum base alloy will increase its strength. For example, British Patent No. 1,089,454 teaches the use of an aluminum base alloy containing 5 to 7% copper and 0.1 to 0.5% magnesium as well as an optional silver additive to obtain an alloy suitable for use in aero-engine components operating in elevated temperatures when the alloy is in the wrought or worked state and has been subsequently heat treated, quenched and then artificially aged for 5 to 36 hours between 170.degree. and 250.degree. C. Criner U.S. Pat. No. 2,784,126, assigned to the assignee of this invention, teaches a high strength aluminum base alloy suitable for use in internal combustion engines. This alloy contains 0.05 to 0.70% magnesium and 5 to 13% copper as well as manganese, vanadium and zirconium. In addition, an alloy registered with the Aluminum Association as 2001 for use as containers for bottled gas contains 5.2 to 6% copper, 0.2 to 0.45 magnesium and 0.15 to 0.50 manganese, plus impurities.
Levy U.S. Pat. No. 3,826,688 describes an Al--Cu--Mg alloy product having up to about 5% copper and up to about 2% magnesium to achieve a substantially single phase structure to improve the fracture toughness of the resulting alloy product after heating treatment, working and aging.
It is also known that cold work after solution heat treatment and quenching can increase the strength of Mg-free Al--Cu alloys, e.g., Aluminum Association alloy (AA) 2219, and of Al--Cu--Mg alloys containing more than about 1.2% magnesium, e.g. AA2024. This effect is evident in naturally aged (T3) and artificially aged (T8) tempers. This effect of cold work after solution heat treatment on increasing strength, however, is minimal or nonexistent in Al--Cu--Mg alloys, e.g., AA2014 and AA2017, containing magnesium at 0.2 to 0.8% levels, but low copper levels, i.e., about 4.5%. Moreover, cold work before artificial aging decreases the strength of a Mg-free Al--Cu alloy containing small amounts of certain elements, such as Sn or Cd, e.g., AA2021.
However, while some of these combinations of copper and magnesium in an aluminum base alloy will contribute to increases in strength, it is also known that the presence of high amounts of both copper and magnesium, in combination, can render the alloy difficult to cast as well as interfering with its weldability.
Quite surprisingly, in view of the prior art metallurgical literature, it has now been discovered, however, that an aluminum base alloy product containing copper and small controlled amounts of magnesium and utilizing critical cold working and aging treatments may be produced having suitable stress-corrosion cracking resistance, high strength rendering it suitable for use as armor for ballistics protection, as well as acceptable weldability and castability.